This invention relates to a process for purification of an impure feedstock containing an alkyl alkanoate which contains up to 12 carbon atoms as well as at least one impurity selected from an aldehyde and a ketone and containing the same number of carbon atoms as the alkyl alkanoate.
Alkyl alkanoates can be produced by esterification of an alkaroic acid with an alkanol. An example is the esterification of acetic acid with ethanol according to equation (1):
CH3.CO.OH+CH3CH2OH=CH3.CO.O.CH2.CH3+H2Oxe2x80x83xe2x80x83(1)
Because the esterification reaction does not tend to lead to formation of by-products which have boiling points close to that of the alkyl alkanoate, recovery of substantially pure alkyl alkanoate from the esterification product mixture is usually not complicated by the presence of by-products of the esterification reaction.
Alkyl alkanoates can alternatively be produced using the Tischenko reaction. For example ethyl acetate can be produced from acetaldehyde according to the Tischenko reaction given in equation (2):
2CH3.CHO=CH3.CO.O.CH2.CH3xe2x80x83xe2x80x83(2).
It is also possible to produce alkyl alkanoates from alkanols by dehydrogenation. For example ethyl acetate can be made from ethanol by dehydrogenation according to equation (3):
2CH3.CH2.OH=CH3.CO.O.CH2.CH3+2H2xe2x80x83xe2x80x83(3).
Catalytic dehydrogenation of alcohols with reduced copper under ultra violet light was described by S. Nakamura et al, in Bulletin of the Chemical Society of Japan (1971), Vol. 44, pages 1072 to 1078.
K. Takeshita et al described reduced copper catalysed conversion of primary alcohols into esters and ketones in Bulletin of the Chemrical Society of Japan, (1978) Vol. 51(9), pages 2622 to 2627. These authors mention that the mechanism for ester formation has been described in the literature as the Tischenko reaction. That is to say that dehydrogenation of ethanol yields acetaldehyde as an intermediate which combines according to the Tischenko reaction to produce ethyl acetate. Alternatively, or as well, 1 mole of ethanol may combine with 1 mole of acetaldehyde to yield 1 mole of ethyl acetate and 1 mole of hydrogen according to equation (4):
CH3CH2OH+CH3.CHO=CH3.CO.O.CH2.CH3+H2xe2x80x83xe2x80x83(4).
Production of esters from primary alcohols by dehydrogenation using bromous acid or a salt thereof in acid medium is described in JP-A-59/025334.
In SU-A-362814 there is described a process for production of ethyl acetate by dehydrogenation of ethanol at 180xc2x0 C. to 300xc2x0 C. in the presence of a copper catalyst containing zinc as an activator with an ethanol feed rate of 250 to 700 liters per liter of catalyst per hour.
The dehydrogenation of ethanol to form ethyl acetate is described in GB-A-287846. This proposes use of a dehydrogenating agent, such as a copper catalyst, a temperature of from 250xc2x0 C. to 500xc2x0 C., and a pressure of more than 10 atmospheres (1.013xc3x97106 Pa)
Vapour phase contact of ethanol at a temperature above its critical temperature with a catalyst comprising copper and a difficultly reducible oxide, such as zinc oxide or manganese oxide, is proposed in GB-A-312345 for the production of ethyl acetate, use of a temperature of 375xc2x0 C. and a pressure of 4000 psi (27.58 Mpa) being suggested.
GB-A-470773 teaches a process for conversion of ethanol to ethyl acetate by dehydrogenating ethanol over a catalyst consisting of a reduced metal, for example, copper on infusorial earth with 10% uranium oxide as promoter, maintained at a temperature of 220xc2x0 C. to 260xc2x0 C. , removing by condensation some of the gas-vapour product rich in hydrogen resulting from the reaction, and returning the gaseous remainder rich in hydrogen to the catalysing zone.
EP-A-0151886 describes a process for the preparation of C2+ esters of alkyl carboxylic acids from C2+ primary alcohols which comprises contacting a vaporous mixture containing a primary C2+ alkanol and hydrogen in an alkanol:hydrogen molar ratio of from 1:10 to about 1000:1 at a combined partial pressure of alkanol and hydrogen of from about 0.1 bar (103 Pa) up to about 40 bar (4xc3x97106 Pa) and at a temperature in the range of from about 180xc2x0 C. to about 300xc2x0 C. in a catalytic reaction zone with a catalyst consisting essentially of a reduced mixture or copper oxide and zinc oxide, and recovering a reaction product mixture containing a primary C2+ alkyl ester of an alkyl carboxylic acid which ester contains twice as many carbon atoms as the primary C2+ alkanol.
In EP-A-0201105 there is described a method for converting primary alcohols, such as ethanol, to their corresponding alkanoate esters which involves the regulation of the mole feed ratio of hydrogen gas to alkanol in the reaction zone of a copper chromite containing catalyst.
Separation of ethyl acetate from a composition comprising ethyl acetate, ethanol and water is disclosed in JP-A-05/186392 by feeding the composition to a distillation column to obtain a quasi-azeotropic mixture comprising ethyl acetate, ethanol and water, condensing it, separating the condensate into an organic layer and an aqueous layer, returning the organic layer to the column, and recovering ethyl acetate as a bottom product from the column.
EP-A-0331021 describes how carbonylation of olefins to produce monocarboxylate esters causes formation of aldehydes and acetals as byproducts. Monocarboxylate esters produced in this way are subjected to a three step purification process involving treatment with a strongly acidic agent, followed by hydrogenation and distillation. The initial treatment with a strongly acidic agent is intended to convert acetals to vinyl ethers and aldehydes and acetals to aldols. The subsequent hydrogenation step then converts these compounds to byproducts which are more easily separated from the desired monocarboxylate ester.
EP-A-0101910 contains a similar disclosure regarding carbonylation of olefins to give monocarboxylate esters. It proposes treatment of the monocarboxylate ester with hydrogen at elevated temperature in the presence of an acidic ion exchanger or zeolite doped with one or more metals of Group VIII of the Periodic Table, followed by hydrogenation. It is stated that acetals present as byproducts are converted to vinyl ethers which are converted by hydrogenation to low boiling esters or the aldehydes and acetals are converted to high boilers by an aldol reaction. Unsaturated ketones are converted to saturated ketones.
One particular problem in production of alkyl alkanoates by dehydrogenation of an alkanol is that the reaction product mixture tends to be a complex mixture including esters, alcohols, aldehydes and ketones. The reaction product mixtures contain components with boiling points close to that of the desired alkyl alkanoate or alkanoates. In some cases such components can form azeotropes, including azeotropes with the desired alkyl alkanoate or alkanoates whose boiling points are close to that of the alkyl alkanoate or alkanoates. This is a particular problem when a high purity alkyl alkanoate, such as ethyl acetate, is desired.
The present invention accordingly seeks to provide a novel process for recovery of a substantially pure alkyl alkanoate from an impure feedstock, for example a crude product produced by dehydrogenation of an alkanol which contains by-products whose boiling point is close to that of the desired alkyl alkanoate or alkanoates and which, in some cases at least, from azeotropes with the alkyl alkanoate or alkanoates whose boiling points are close to that of the desired alkyl alkanoate or alkanoates. It further seeks to provide a process for purification of an impure feedstock containing an alkyl alkanoate containing up to 12 carbon atoms which further contains as an impurity at least one aldehyde and/or ketone which contains the same number of carbon atoms as the alkyl alkanoate so as to result in production of a substantially pure alkyl alkanoate product. In addition the invention seeks to provide an improved process for the production of an alkyl alkanoate by dehydrogenation or oxidation of an alkanol, by reaction of an alkanol with an alkanal, or by oxidation of an alkanol to an alkanal followed by the Tischenko reaction which enables production of a substantially pure alkyl alkanoate product, despite the presence in the crude reaction product of aldehydes and ketones which would otherwise contaminate the alkyl alkanoate product.
According to the present invention there is provided a process for the purification of an impure feedstock comprising an alkyl alkanoate which contains up to 12 carbon atoms which comprises:
(a) providing an impure feedstock containing an alkyl alkanoate which contains up to 12 carbon atoms, said feedstock further containing at least one impurity which is selected from an aldehyde and a ketone and which contains the same number of carbon atoms as said alkyl alkanoate;
(b) contacting said impure feedstock with a selective hydrogenation catalyst in the presence of hydrogen in a selective hydrogenation zone maintained under selective hydrogenation conditions effective for selective hydrogenation of impurities containing reactive carbonyl groups thereby to hydrogenate said impurities to the corresponding alcohols;
(c) recovering from the selective hydrogenation zone a selectively hydrogenated reaction product mixture comprising said alkyl alkanoate, hydrogen, and said corresponding alcohols;
(d) distilling material of the selectively hydrogenated reaction product mixture in one or more distillation zones so as to produce substantially pure alkyl alkanoate therefrom; and
(e) recovering said substantially pure alkyl alkanoate.
The invention further provides a process for the production of an alkyl akanoate containing up to 12 carbon atoms by dehydrogenation of an alkanol which comprises: (i) contacting a vaporous mixture containing an
alkanol and hydrogen with a dehydrogenation catalyst in a dehydrogenation zone maintained under dehydrogenation conditions effective for dehydrogenation of an alkanol to yield an alkyl alkanoate containing up to 12 carbon atoms;
(ii) recovering from the dehydrogenation zone an intermediate reaction mixture comprising hydrogen and liquefiable products comprising said alkyl alkanoate, said alkanol, hydrogen and by-products containing reactive carbonyl groups; and
(iii) subjecting at least a portion of the liquefiable products of the intermediate reaction product mixture as impure feedstock to a process as outlined in the preceding paragraph.
The impure feedstock may be effectively any feedstock which contains an alkyl alkanoate, such as ethyl acetate, or a mixture of alkyl alkanoates, possibly water, an alkanol, such as ethanol, or a mixture of alkanols, and minor amounts of impurities including aldehydes and/or ketones. In the case of ethyl acetate such aldehydes and ketones include n-butyraldehyde, acetone and butan-2-one. Example of such an impure feedstock are the intermediate reaction product mixtures obtained by dehydrogenation of an alkanol, such as ethanol, or of a mixture of alkanols, such as ethanol and iso-butanol.
A range of undesirable impurities may be present in the feedstock, some of which would cause separation problems if the feedstock were to be directly refined because their boiling points are close to that of the alkyl alkanoate or because, in some cases at leas, they form azeotropes with the alkyl alkanoate whose boiling point is close to that of the alkyl alkanoate. For example, purification of the specified exemplary alkyl alkanoates can be complicated by the presence of the impurities set out in the following Table 1, the same impurities generally giving rise to problems with all alkyl alkanoates with the same number of carbon atoms.
It will be appreciated by those skilled in the art that Table 1 lists only some of the possible alkyl alkanoates whose production is embraced within the teachings of the present invention. For example, isomeric alkyl alkanoates derived from alkanols and/or alkanoic acids with branched chains can also be mentioned.
Preferably the alkyl alkanoate is a C2 to C4 alkyl ester of a C2 to C4 alkanoic acid, for example, ethyl acetate, n-propyl propionate, or n-butyl butyrate.
For convenience the process will hereafter be described in relation to Durification of impure ethyl acetate feedstocks.
In the case of an impure feedstock resulting from dehydrogenation of ethanol, the ethanol feedstock may contain impurities or impurities may be formed as by-products in the production of the alkyl alkanoate, for example, in the course of the dehydrogenation step. Problematical impurities are aldehydes and ketones, such as n-butyraldehyde and butan-2-one in the case of ethyl acetate. In order to minimise problems due to the presence of such impurities in the distillation step (d) even in amounts as small as about 0.1 mol % or less, e.g. about 0.01 mol % or less, problematical impurities are substantially removed as a result of the selective hydrogenation step (b). Accordingly, the impure feedstock is contacted in admixture with hydrogen in step (b) with a selective hydrogenation catalyst. The catalyst type and reaction conditions are chosen so that aldehydes and ketones are hydrogenated to their respective alcohols, while hydrogenation of the alkyl alkanoate, e.g. ethyl acetate, is minimal. Among aldehyde and ketone impurities which may be present in an impure ethyl acetate feedstock, butan-2-one and n-butyraldehyde, in particular, would otherwise cause problems in any subsequent distillation. These compounds are hydrogenated in the selective hydrogenation zone in step (b) to the corresponding alcohols, i.e. 2-butanol and n-butanol respectively, which can be readily separated from ethyl acetate by distillation.
The mixture supplied to the selective hydrogenation zone in step (b) contains, in addition to ethanol, hydrogen either alone or in admixture with one or more inert gases that are inert to the reactants and catalysts in the selective hydrogenation step (b) of the process of the invention. Examples of such inert gases are nitrogen, methane, and argon. The source of the hydrogen used in the selective hydrogenation step (b) may be hydrogen formed in the dehydrogenation step and accordingly may include gas recycled from the downstream end of the selective hydrogenation zone as described further below.
The selective hydrogenation step (b) is typically conducted at a temperature of from about 40xc2x0 C. to about 120xc2x0 C., preferably at a temperature in the range of from about 60xc2x0 C. to about 80xc2x0 C.
Typical selective hydrogenation conditions include use of a feedstock:hydrogen molar ratio of from about 1000:1 to about 5:1, for example about 20:1.
The combined partial pressure of feedstock and hydrogen in the selective hydrogenation zone typically lies in the range of from about 5 bar (5xc3x97105 Pa) up to about 80 bar (8xc3x97106 Pa), and is even more typically about 24 bar (2.5xc3x97106 Pa) to about 50 bar (5xc3x97106 Pa)
The selective hydrogenation catalyst used in step (b) of the process of the invention is selected to have good activity for hydrogenation of reactive carbonyl containing compounds, but relatively poor ester hydrogenation activity. Suitable catalysts comprise metals selected from nickel, palladium and platinum. Ruthenium, supported on carbon, alumina or silica is also effective, as are other metal catalysts such as rhodium and rhenium. Preferred catalysts include nickel on alumina or silica and ruthenium on carbon. Particularly preferred catalysts include 5% ruthenium on carbon available from Engelhard.
The rate of supply of impure feedstock to the selective hydrogenation zone typically corresponds to a liquid hourly space velocity (LHSV) of from about 0.1 hrxe2x88x921 to about 2.0 hrxe2x88x921, preferably from about 0.2 hr31 1 to about 1.5 hrxe2x88x921. When using a nickel containing catalyst the LHSV may be, for example, from about 0.3 hrxe2x88x921 to about 0.5 hrxe2x88x921.
Step (c) of the process of the present invention comprises recovering from the selective hydrogenation zone a selectively hydrogenated reaction product mixture comprising alkyl alkanoate (e.g. ethyl acetate), alkanol (e.g. ethanol), hydrogen and hydrogenated impurities. Typically this includes a condensation stem in order to separate liquefiable materials from a gaseous stream containing unreacted hydrogen which can be recycled for dehydrogenation or for selective hydrogenation.
The impure feedstock typically contains water and alkanol (e.g. ethanol) in addition to alkyl alkanoate (e.g. ethyl acetate). In this case step (d) of the process of the invention comprises distilling material of the selectively hydrogenated reaction product mixture in one or more distillation zones. When the alkyl alkanoate is ethyl acetate, distillation is effected so as to produce a first composition comprising substantially pure ethyl acetate and a second composition comprising ethanol and water. In this step the selectively hydrogenated reaction product mixture subjected to distillation typically has a water content of less than about 20 mol %, more usually not more than about 15 mol %.
Ethanol, water and ethyl acetate form a minimum boiling ternary azeotrope upon distillation thereof.
One method of separating ethyl acetate from ethanol and water involves extractive distillation with an extractive agent comprising polyethylene glycol and dipropylene glycol, diethylene glycol, or triethylene glycol as described in U.S. Pat. No. 4569726 or with an extractive agent containing dimethyl sulfoxide as described in U.S. Pat. No. 4379028. Hence step (d) may comprise an extractive distillation procedure.
Preferably, however, distillation is carried out in step (d) by a procedure which takes advantage of the fact that the composition of the minimum boiling ternary azeotrope formed by ethanol, water and ethyl acetate depends upon the pressure at which distillation is effected. Hence a preferred distillation procedure comprises supplying material of the selectively hydrogenated reaction product mixture to a first distillation zone maintained under distillation conditions effective for distillation therefrom of a first distillate comprising ethyl acetate, ethanol, and water, recovering a first distillate comprising ethyl acetate, ethanol, and water from the first distillation zone and a bottom product comprising ethanol and water, supplying material of the first distillate to a second distillation zone maintained under distillation conditions effective for distillation therefrom of a second distillate comprising ethanol, water, and ethyl acetate (preferably a minor amount of ethyl acetate) and so as to yield a substantially pure ethyl acetate bottom product, and recovering a substantially pure ethyl acetate bottom product from the second distillation zone. The first distillation zone is preferably operated at a pressure less than about 4 bar (4xc3x97105 Pa), preferably from about 1 bar (105 Pa) up to about 2 bar (2xc3x97105 Pa), while the second distillation zone is operated at a higher pressure than that of the first distillation zone, for example at a pressure of from about 4 bar (4xc3x97105 Pa) to about 25 bar (2.5xc3x97106 Pa), preferably from about 9 bar (9xc3x97105 Pa) to about bar (15xc3x97105 Pa).
It can be shown that in this Dreferred distillation procedure the rate of flow of the first distillate from the first distillation zone to the second distillation zone and the corresponding flow rate from the second distillation zone to the first distillation zone of the second distillate can be minimised by operating one of the distillation zones so that the distillate has a composition very close to that of the ternary azeotrope at that pressure. However, in order to operate that zone so that the distillate has a composition close to that of the ternary azeotrope at its pressure of operation, a high degree of separation is required which necessitates use of a distillation column with many distillation trays and a high heat input. In addition, since water has the highest latent heat of vaporisation out of the three components of the ternary azeotrope, the total heat input to the two zones can be minimised by minimising the water content of the feeds to the distillation zones.
In addition to forming a ternary azeotrope, the three components of the ternary azeotrope can each also form binary azeotropes with one of the other components. For example, ethanol forms a binary azeotrope with water and also with ethyl acetate it is preferred to select a pressure of operation of the second distillation zone so that the binary azeotrope between ethanol and ethyl acetate at that pressure has a lower ethyl acetate content than the ternary azeotrope at that pressure and further to select a pressure of operation for the first distillation zone and to adjust the flow rates of the distillates between the first and second zones so that the first distillate has as low a water content as possible. In this way the second distillate recovered from the second distillation zone low content of ethyl acetate.
In the preferred distillation procedure an ethanol rich stream containing substantially all of the water in the selectively hydrogenated reaction mixture is recovered from the bottom of the first distillation zone, while an overhead stream that contains xe2x80x9clightxe2x80x9d components present in the selectively hydrogenated reaction product mixture is recovered from the first distillation zone, and the first distillate comprises a liquid draw stream which is recovered from an upper region of the first distillation zone and which comprises ethyl acetate, ethanol, water and minor amounts of other components. By the term xe2x80x9clightxe2x80x9d components is meant components that have lower boiling points than ethyl acetate and its azeotropes with water and ethanol. The liquid draw stream typically contains less than about mol 10% water. For example, it suitably comprises from about 1 mol % to about 6 mol % water, from about 40 mol % to about 55 mol % ethyl acetate, not more than about 2 mol a minor products (preferably not more than about 1 mol % minor products) and the balance ethanol. Thus it may typically contain about 45 mol % ethyl acetate, about 50 mol % ethanol, about 4 mol % water and about 1 mol % other components. This liquid draw stream is passed to the second distillation zone. The second distillate, with a typical composition of about 25 mol % ethyl acetate, about 68 mol % ethanol, about 6 mol % water, and about 1 mol % of other components, is recovered as an overhead stream from the second distillation zone, while a bottom product comprising ethyl acetate is recovered from the second distillation zone which typically contains from about 99.8 mol % to about 99.95 mol % ethyl acetate; this second distillate is returned to the first distillation zone, preferably at a point above the feed point of the liquefiable products of the selectively hydrogenated reaction product mixture.
The overhead stream from the first distillation zone contains xe2x80x9clightxe2x80x9d components present in the intermediate reaction product mixture, such as diethyl ether, acetaldehyde and acetone. It can be burnt as a fuel.
In this preferred process of the invention the ethanol rich stream recovered from the bottom of the first distillation zone can, if desired, be subjected to treatment for the removal of water therefrom thereby to produce a relatively dry ethanol stream which can be used for a purpose which will be described below, if desired. products, including unknown products, with high boiling points compared to those of ethanol and ethyl acetate. These can be separated from the ethanol and water by distillation, if desired, prior to effecting removal of water from the resulting distillate. The resulting ethanol stream, after water removal, can be recycled for production of further ethyl acetate.
One suitable method for removal of water from the ethanol rich stream or from the distillate resulting from xe2x80x9cheaviesxe2x80x9d removal is molecular sieve adsorption. Azeotropic distillation with a suitable entrainment agent, such as benzene or cyclohexane, can alternatively be used. Membranes are currently under development which will enable separation of water from ethanol; these are reported to be nearly ready for commercial exploitation. Hence use of a membrane is another option available for separating water from the ethanol rich stream.
Preferably the water content of the thus produced relatively dry ethanol is less than about 5 mol %, and preferably less than about 2 mol %.
The impure alkyl alkanoate feedstock may, for example, comprise liquefiable components of a reaction product mixture produced by dehydrogenation of ethanol. Such ethanol may have been produced by hydration of ethylene, by the Fischer Tropsch process, or by fermentation of a carbohydrate source, such as starch (for example, in the form of a corn steep liquor) It may alternatively be a by-product of another industrial process. It may contain, besides ethanol, minor amounts of water as well as small amounts of impurities resulting from by-product formation during its synthesis. If there is provision for recycle of recovered ethanol, then any by-products formed during production of ethyl acetate will contribute to the level of by-products present in the feedstock. Impurities present in the ethanol feedstock may include, for example, higher alcohols such as n-propanol, iso-propanol, n-butanol and sec-pentanol; ethers, such as diethyl ether, and di-iso-propyl ether; esters, such as iso-propyl acetate, sec-butyl acetate and ethyl butyrate; and ketones, such as acetone, butan-2-one, and 2-pentanone. At least some of these impurities can be difficult to remove from ethyl acetate, even when they are present in quantities as low a about 0.1 mol % or less, by traditional distillation procedures because they have boiling points which are close to that of ethyl acetate and/or form distillates therewith.
In the dehydrogenation step ethanol can be converted to ethyl acetate by a dehydrogenation procedure which comprises contacting a vaporous mixture containing ethanol and hydrogen with a dehydrogenation catalyst in a dehydrogenation zone maintained under dehydrogenation conditions effective for dehydrogenation of ethanol to yield ethyl acetate.
Typical dehydrogenation conditions include use of an ethanol:hydrogen molar ratio of from about 1:10 to about 10000:1, a combined partial pressure of ethanol and hydrogen of up to about 50 bar (5xc3x97106 Pa), and a temperature in the range of from about 100xc2x0 C. to about 260xc2x0 C.
Preferably the combined partial pressure of ethanol and hydrogen ranges from about 3 bar (3xc3x97105 Pa) up to about 50 bar (5xc3x97106 Pa), and is more preferably at least 6 bar (6xc3x97105 Pa) up to about 30 bar (3xc3x97106 Pa), and even more preferably in the range of from about 10 bar (106 Pa) up to about 20 bar (2xc3x97106 Pa), for example about 12 bar (1.2xc3x97106 Pa).
Dehydrogenation is preferably conducted in the dehydrogenation zone at a temperature of from about 200xc2x0 C. to about 250xc2x0 C., preferably at a temperature in the range of from about 210xc2x0 C. to about 240xc2x0 C., even more preferably at a temperature of about 220xc2x0 C.
The ethanol:hydrogen molar ratio in the vaporous mixture fed into contact with the dehydrogenation catalyst usually will not exceed about 400:1 or about 500:1 and may be no more than about 50:1.
The dehydrogenation catalyst is desirably a catalyst containing copper, optionally in combination with chromium, manganese, aluminum, zinc, nickel or a combination of two or more of these metals, such as a copper, manganese and aluminium containing catalyst. Preferred catalysts comprise, before reduction, copper oxide on alumina, an example of which is the catalyst sold by Mallinckrodt Specialty Chemicals, Inc., under the designation E408Tu, a catalyst which contains 8% by weight of alumina. Other preferred catalysts include chromium promoted copper catalysts available under the designations PG85/1 (Kvaerner Process Technology Limited) and CU0203T (Engelhard), manganese promoted copper catalysts sold under the designation T4489 (Sxc3xcd Chemie AG), and supported copper catalysts sold under the designation D-32-J (Sxc3xcd Chemie AG). E408Tu is a particularly preferred dehydrogenation catalyst.
In the dehydrogenation step the rate of supply of the ethanol feedstock to the dehydrogenation zone typically corresponds to an ethanol liquid hourly space velocity (LHSV) of from about 0.5 hrxe2x88x921 to about 1.0 hrxe2x88x921.
Hydrogen is produced as a result of the dehydrogenation reaction and can be recycled to the dehydrogenation zone from downstream in the process. The hydrogen can be substantially pure hydrogen or can be in the form of a mixture with other gases that are inert to the ethanol feedstock and to the dehydrogenation catalyst. Examples of such other gases include inert gases such as nitrogen, methane and argon.
In the dehydrogenation zone, side reactions may also occur, including formation of water. It is postulated that such side reactions, in the case of production of ethyl acetate, include formation of acetaldehyde which in turn can undergo aldol formation, followed by dehydration to form an unsaturated alcohol and water. These reactions can be summarised thus:
CH3CH2OH=CH3CHO+H2xe2x80x83xe2x80x83(5)
2CH3CHO=CH3CH(OH)CH2CHOxe2x80x83xe2x80x83(6) and
CH3CH(OH)CH2CHO=CH3CH=CHCHO+H2Oxe2x80x83xe2x80x83(7).
The crotonaldehyde produced by equation (7) can then undergo hydrogenation to form n-butanol thus:
CH3CH=CHCHO+H2=CH3CH2CH2CH2HO.xe2x80x83xe2x80x83(8)
Other side reactions which release water as a by-product include formation of ketones, such as acetone and butan-2-one, and formation of ethers, such as diethyl ether.
In such a dehydrogenation process there is recovered from the ethyl acetate production zone an intermediate reaction product mixture comprising hydrogen and liquefiable products comprising ethyl acetate, ethanol, hydrogen and by-products containing reactive carbonyl groups; this intermediate reaction product mixture can be used as impure feed to the recovery process of the invention. The step of recovering this intermediate reaction product mixture can be effected in any convenient manner and may include a condensation step in order to condense liquefiable products present in the intermediate reaction product mixture. Alternatively the intermediate reaction product can be passed directly to step (b) without any intermediate condensation step.
The production of a relatively dry ethanol stream has been mentioned above. This can be recycled, if desired, to the dehydrogenation step, if used, or can be used for any other desired purpose.